Arsenic Stress in Plants: An Inside Story - Crop Improvement: New Approaches and Modern Techniques - page 391

can suppress the arsenic sensitivity. The expression of PvACR3 in Pteris vittata is
rapidly increased within 24 h after exposure to arsenate. But to date, there is lack
of information regarding these type of transporters mediated arsenic detoxification
in higher plants and effect on their expression in arsenic toxicity. Little informa-
tion are present upon the expression of these transporter from the study of whole
genome analysis in rice that three genes with the predicted putative function of an
ABC transporter family protein (Os 04g49890, Os04g52900, and Os11g05700) is
up regulated in the arsenic toxicity. Phylogenetic analysis of these ABC transporters
gene sequences revealed that they are also MRP transporter. MRPs are a subclass
of that ATP-binding cassette (ABC) transporter, which are involved in the transport
of glutathione conjugated compounds into the vacuoles of plants (Rea et al. 1998 ).
And also a single gene with an annotated putative function of a glutathione conju-
gate transporter (Os04g 1321 0) is also upregulated on exposure to arsenic (Norton
et al. 2008 ).
A number of genes involved in N transport also appear to alter expression in
response to As. Amino acid transporters are down- regulated in response to As(V)
in roots and seedlings of rice (Norton et al. 2008 ; Chakrabarty et al. 2009 ). How-
ever, amino acid transporter gene transcript levels were not influenced by As(III)
(Chakrabarty et al. 2009 ). Peptide and oligopeptide transporters have also been re-
ported to be As(V) responsive in rice, but reports disagree on the direction (Norton
et al. 2008 ; Chakrabarty et al. 2009 ).
Arsenic is also known to interfere with sulfur metabolism. It is reported at mo-
lecular level that in As(V)-treated rice, up to five sulfate transporter genes are up-
regulated in roots (Norton et al. 2008 ), and at least one sulfate transporter is up-reg-
ulated in Arabidopsis (Sung et al. 2009 ). As(III) also induces a sulfate transporter
gene in rice and B. juncea seedlings (Chakrabarty et al. 2009 ). Although it is not yet
clear whether As(V) and As(III) influence the expression of these transporters in the
same way, but at least one of the transporter genes is induced by both forms of As
(Chakrabarty et al. 2009 ). During transfer of sulfate from soil to plant once sulfate
is reduced to sulfide, the sulfide is combined with O -acetylserine to form Cys in a
reaction catalyzed by O -acetylserine(thiol)-lyase also known as Cys synthase. It ap-
pears that As(V) and As(III) exposure may cause a down-regulation of (OAS-TL),
(OAS-TL)in As-sensitive plants. OAS-TL protein disappeared from maize shoots
exposed to As (Requejo and Tena 2006 ), while OAS-Tl activity was repressed in an
As-sensitive line of B. juncea (Srivastava et al. 2009 ).
Moreover, in rice, several methyl transferase genes are induced by As(V)-treat-
ment (Norton et al. 2008 ). Two of these are homocysteine S-methyltransferases,
which catalyze the formation of S -adenosyl-l-homocysteine and Met from S -ade-
nosylmethionine and l-homocysteine.The enzyme is involved in the synthesis of
S -methylmethionine (Ranocha et al. 2001 ), and may play a key role in maintaining
a pool of soluble Met, in the cycling of methyl groups within cells, or as a phloem-
mobile form of Met that can be used to translocate sulfur derived from protein
degradation (Bürstenbinder and Sauter 2012 ).